Create one file out of multiple photos

How do I create a master file to combine multiple photos? (I'm COMPLETELY new to Photoshop) I DO NOT want these photos "on top of each other". I want them all side by side, but merged into one file.

I need step by step instructions, as I've never used Photoshop. The company that I'm ordering a custom photo product from said that I can't upload individual files of all the photos I want included. They said I have to create a master file in Photoshop. When I search for info on creating a master file (search, YouTube, etc), they all mention LAYERING photos, but I need them all laid out next to each other, then saved as a single file.